Thermoplastic elastomer (TPE) compositions based upon polyurethane or polyester elastomers are well known and are used in many industrial applications including wire and cable coverings, e.g., insulated wires and protective cable jackets. To be useful as a wire and cable covering, these polyurethane or polyester based elastomers must exhibit, among other things, good mechanical properties, e.g., good elasticity and tensile strength, extrudability, flame retardancy and competitive cost. These polyurethane and polyester type thermoplastic elastomers inherently possess good elasticity and mechanical strength, but they are not inherently flame retardant and they are not low cost relative to many other polymers, e.g., many polyolefins.
Flame retardancy can be imparted to these elastomers by admixing it with one or more flame retardants. Many flame retardants comprise halogen functionality or one or more halogen compounds. While these additives impart desirable flame-resistant properties to the TPU, they are also prone to release toxic gases upon ignition. Consequently, flame retardants that do not contain halogen functionality or halogen compounds are often favored over those flame retardants that do contain such halogen functionality or compounds.
One such class of halogen-free flame retardants that have been admixed with thermoplastic polyurethanes and polyester elastomers is the intumescent-type flame retardants. A wide range of these intumescent flame retardant additives are available, they typically include phosphorus and/or nitrogen based constituents to provide the char forming flame retardant characteristics. There is good value in identifying co-agents providing synergistic flame retardant benefit allowing a reduced intumescent additive loading, since these intumescent flame retardant materials can be costly and can involve trade-offs on other key properties.
To lower the cost of a cable covering comprising a thermoplastic polyurethane or polyester elastomer, it may be possible to include a lower cost polymer component, e.g., a polyolefin. However, the combination of thermoplastic polyurethane or polyester elastomer, polyolefin and phosphorus-based flame retardant often yields a composition with reduced mechanical properties relative to the TPU alone or the TPU/polyolefin blend, particularly when the phosphorus-based flame retardant is present at a relatively high loading, e.g., in excess of 40 weight percent based on the combined weight of TPU, polyolefin and flame retardant.
Moreover, if the cable covering is colored, then the thermoplastic polyurethane or polyester elastomers are typically also admixed with a colorant, e.g., carbon black for black or titanium dioxide for white. However, these colorants, particularly carbon black, can add to the fuel load of the TPU admixture and thus reduce the flame retardancy of the admixture.
EP 1 719 800 teaches a flame retardant composition comprising piperazine and melamine treated with silicone oil in combination with a (poly/pyro)phosphate compound. The flame retardant composition can be admixed with various synthetic resins including polyolefins, polyurethanes or polyesters.
Patent Application Publication US 2007/0221890 teaches phosphorus-containing compounds that are formed by reacting (A) an organophosphorus compound having a group selected from the group H—P═O, the group P—H and the group P—OH, and (B) a compound of a complex, specified formula. The compounds are useful for imparting flame retardancy to epoxy and polyurethane resins.
Patent Application Publication US 2006/0151758 teaches compositions comprising intumescent flame retardants in combination with a plastic resin. The compositions are useful as wire and cable jacketing and insulation. Plastic resins include polyolefins, engineering resins and thermoset resins.
WO 2007/031450 teaches flame retardant compositions comprising (a) at least one ester of a phosphorus-containing oxo acid, (b) at least one flame retardant component based on a nitrogen compound, e.g., melamine cyanurate, and (c) a polyurethane polymer base. The polyurethane polymer base can contain a polyolefin, and the composition can be used for cable coverings.
U.S. Pat. No. 5,837,760, Patent Application Publication US 2008/0132615 and JP 2001/261855 all teach flame retardant compositions comprising a TPU and a phosphorus-containing compound. JP 2001/261855 also teaches that the composition contains an ethylene-based thermoplastic elastomer, e.g., ethylene vinyl acetate.
Accordingly, of continuing interest is a TPU/olefin-polymer composition that comprises a phosphorus-based flame retardant and that is free of halogen. Moreover, the composition should exhibit sufficiently good mechanical and flame retardant properties, even at high flame retardant loadings, so as to be useful in wire and cable applications.